The C4 photosynthesis bifunctional enzymes, PDRPs, of maize are co-opted to cytoplasmic viral replication complexes to promote infection of a prevalent potyvirus sugarcane mosaic virus.

In maize, two pyruvate orthophosphate dikinase (PPDK) regulatory proteins, ZmPDRP1 and ZmPDRP2, are respectively specific to the chloroplast of mesophyll cells (MCs) and bundle sheath cells (BSCs). Functionally, ZmPDRP1/2 catalyse both phosphorylation/inactivation and dephosphorylation/activation of ZmPPDK, which is implicated as a major rate-limiting enzyme in C4 photosynthesis of maize. Our study here showed that maize plants lacking ZmPDRP1 or silencing of ZmPDRP1/2 confer resistance to a prevalent potyvirus sugarcane mosaic virus (SCMV). We verified that the C-terminal domain (CTD) of ZmPDRP1 plays a key role in promoting viral infection while independent of enzyme activity. Intriguingly, ZmPDRP1 and ZmPDRP2 re-localize to cytoplasmic viral replication complexes (VRCs) following SCMV infection. We identified that SCMV-encoded cytoplasmic inclusions protein CI targets directly ZmPDRP1 or ZmPDRP2 or their CTDs, leading to their re-localization to cytoplasmic VRCs. Moreover, we found that CI could be degraded by the 26S proteasome system, while ZmPDRP1 and ZmPDRP2 could up-regulate the accumulation level of CI through their CTDs by a yet unknown mechanism. Most importantly, with genetic, cell biological and biochemical approaches, we provide evidence that BSCs-specific ZmPDRP2 could accumulate in MCs of Zmpdrp1 knockout (KO) lines, revealing a unique regulatory mechanism crossing different cell types to maintain balanced ZmPPDK phosphorylation, thereby to keep maize normal growth. Together, our findings uncover the genetic link of the two cell-specific maize PDRPs, both of which are co-opted to VRCs to promote viral protein accumulation for robust virus infection.

Fengycin produced by Bacillus subtilis XF-1 plays a major role in the biocontrol of Chinese cabbage clubroot via direct effect and defense stimulation.

Bacillus subtilis XF-1 is a well-investigated biocontrol agent against the biotrophic Plasmodiophora brassicae Woron., the causal agent of clubroot disease of cruciferous crops. The present study demonstrates that XF-1 could efficiently control clubroot disease via leaf spraying and provides an understanding of the biocontrol mechanisms. High-performance thin-layer chromatography (HTPLC) analysis indicated the presence of fengycin-type cyclopeptides in the supernatant. A ppsB deletion mutant of XF-1 resulted in no fengycin production, significantly reduced the lysis rate of testing spores in vitro and the primary infection rate of root hair in vivo, and decreased the protection value against clubroot disease under the greenhouse conditions. Confocal laser scanning microscopy proved that fengycin was not required for leaf internalization and root colonization. Moreover, the expression level of the ppsB gene in XF-1 was regulated by its cell density in root during interaction with P. brassicae. In addition, the ΔppsB mutant of XF-1 could not efficiently control disease because it led to a lower activation level of the jasmonic acid and salicylic acid signaling pathways in roots, which are necessary for the plant defense reaction upon pathogen invasion. Altogether, the present study provides a new understanding of specific cues in the interaction between B. subtilis and P. brassicae as well as insights into the application of B. subtilis in agriculture.

Maize miR167-ARF3/30-polyamine oxidase 1 module-regulated H2O2 production confers resistance to maize chlorotic mottle virus.

Maize chlorotic mottle virus (MCMV) is the key pathogen causing maize lethal necrosis (MLN). Due to the sharply increased incidence of MLN in many countries, there is an urgent need to identify resistant lines and uncover the underlying resistance mechanism. Here, we showed that the abundance of maize (Zea mays) microR167 (Zma-miR167) positively modulates the degree of resistance to MCMV. Zma-miR167 directly targets Auxin Response Factor3 (ZmARF3) and ZmARF30, both of which negatively regulate resistance to MCMV. RNA-sequencing coupled with gene expression assays revealed that both ZmARF3 and ZmARF30 directly bind the promoter of Polyamine Oxidase 1 (ZmPAO1) and activate its expression. Knockdown or inhibition of enzymatic activity of ZmPAO1 suppressed MCMV infection. Nevertheless, MCMV-encoded p31 protein directly targets ZmPAO1 and enhances the enzyme activity to counteract Zma-miR167-mediated defense to some degree. We uncovered a role of the Zma-miR167-ZmARF3/30 module for restricting MCMV infection by regulating ZmPAO1 expression, while MCMV employs p31 to counteract this defense.

Biocontrol of plant pathogens in omics era-with special focus on endophytic bacilli.

Nearly all plants and their organs are inhabited by endophytic microbes which play a crucial role in plant fitness and stress resilience. Harnessing endophytic services can provide effective solutions for a sustainable increase in agriculture productivity and can be used as a complement or alternative to agrochemicals. Shifting agriculture practices toward the use of nature-based solutions can contribute directly to the global challenges of food security and environmental sustainability. However, microbial inoculants have been used in agriculture for several decades with inconsistent efficacy. Key reasons of this inconsistent efficacy are linked to competition with indigenous soil microflora and inability to colonize plants. Endophytic microbes provide solutions to both of these issues which potentially make them better candidates for microbial inoculants. This article outlines the current advancements in endophytic research with special focus on endophytic bacilli. A better understanding of diverse mechanisms of disease control by bacilli is essential to achieve maximum biocontrol efficacy against multiple phytopathogens. Furthermore, we argue that integration of emerging technologies with strong theoretical frameworks have the potential to revolutionize biocontrol approaches based on endophytic microbes.

Complete genome sequence analysis of a novel citlodavirus isolated from the leaves of Myrica rubra in Yunnan.

Through high-throughput sequencing, a novel citlodavirus, tentatively named "Myrica rubra citlodavirus 1" (MRV1, accession no. OP374189), was isolated from the leaves of Myrica rubra in Yunnan exhibiting narrow deformity of leaf tips, shrinkage, and chlorosis along the veins. The complete genome sequence was determined and analyzed using cloning and Sanger sequencing. MRV1 is a single-stranded circular non-enveloped DNA virus with a genome size of 3775 nucleotides and contains six open reading frames (ORFs). The virion-sense genome strand encodes a coat protein (CP, nt 750-1,493, 247 aa), two hypothetical movement proteins (V3, nt 382-666, 94 aa; and V2, nt 461-895, 144 aa), and one movement protein (MP, nt 1,527-2,438, 303 aa). The complementary strand of the genome encodes two replication proteins (RepA, nt 3,712-2,834, 292 aa; Rep, nt 2,867-2,553, 104 aa). The MRV1 genome contains the stem-loop motif 5'-TAATATTAC-3', which is a highly conserved nonanucleotide motif found in the origin of virion-strand replication in geminiviruses. Genome sequence alignment analysis showed that citrus chlorotic dwarf associated virus (CCDaV, accession no. JQ920490) shared the highest nucleotide sequence similarity (66.10% identity) with MRV1. Phylogenetic analysis showed that CCDaV is the closest known relative of MRV1, and that these viruses clustered in a single branch within a clade consisting of citlodaviruses. These results indicate that MRV1 should be regarded as a new species of the genus Citlodavirus in the family Geminiviridae.

First report of Aloe root and stem rot caused by Phytophthora palmivora in Yunnan Province, China.

Aloe genus plants are perennial evergreen herb belonging to Liliaceae family which is widely used in food, medicine, beauty, and health care (Kumar et al. 2019). In August 2021, symptoms of root and stem rot was observed in approximately 20% of Aloe vera plantings in Yuanjiang County, Yunnan Province, China (23° 64' 53" N, 101° 99' 84" E). The most typical symptoms were stem and root rot, browning and necrosis of vascular tissues, gradual greening, and reddish-browning of leaves from bottom to top, abscission, and eventual plant death (Fig. S1). Therefore, to isolate and identify the pathogen, the plants showing the above symptoms were collected. The plant tissues were cut from the edges of root and stem lesions, followed by disinfection with 75% ethanol for 1 min, rinsed three times with sterilized distilled water, and cut into 3 × 3 mm small squares after excision of marginal tissues. The tissues were transferred to the oomycetes selective medium (Liu et al. 2022) and incubated at 28 °C in the dark for 3~5 days, and suspected colonies were purified. The colonies were then inoculated onto potato dextrose agar (PDA), V8-juice agar (V8), and oatmeal agar (OA) medium plates for morphological characteristics. Finally, 18 isolates with the same colonial and morphological characteristics were obtained from 30 lesioned tissue and one of them was named as ARP1. On PDA, V8 and OA medium plates, the ARP1 colonies were white. On PDA plate, the mycelia were dense and the colonies were petal-like; on V8 plate, the mycelia were cashmere and the colonies were radial or star-like. Whereas, on OA plate, the mycelia were cotton-like and the colonies were fluffy and radial (Fig. S2 A~C). Mycelium did not have septum with high branching and swelling. Sporangia were abundant, semi-papillate, varying in shape from ovoid-ellipsoid to long-ellipsoid, 18-26 × 45-63 µm (average: 22 × 54 µm, n = 30), sporangia released numerous zoospores from the papillate after maturation. The chlamydospores were spherical, 20-35 µm in diameter (average: 27.5 µm, n = 30) (Fig. S2 D~F). These morphological features were like those of the pathogenic species of the oomycetes (Chen et al. 2022). For the molecular characterization, the genomic DNA of the isolate was extracted using the cetyl trimethyl ammonium bromide method, and the translation elongation factor 1α (tef-1α) (Stielow et al. 2015), ß-tubulin (ß-tub) (Kroon et al. 2004) and internal transcribed spacer (ITS) (White et al. 1990) of isolated strain ARP1 were amplified using primer pairs EF1-1018F/EF1-1620R, TUBUF2/TUBUR1 and ITS1/ITS4, respectively. The tef-1α, ß-tub genes and ITS region of ARP1 were directly sequenced and their sequence information was deposited in GenBank under accession numbers OQ506129, OQ506127 and OQ449628. ARP1 was clustered on the same evolutionary branch with Phytophthora palmivora (Fig. S3). To confirm the pathogenicity of ARP1, the main root of A. vera was wounded to 1 cm long and 2 mm deep with a scalpel blade followed by inoculation with 50 ml suspension of ARP1 zoospores at a concentration of 1 × 106 spores / ml per potted plant, and an equal volume of water as control. All inoculated plants were placed in the greenhouse at 28°C, 12 h / 12 h light / dark. After 15 dpi, the inoculated plants showed typical symptoms of wilted and drooping leaves and stem and root rot, same as observed in the field condition (Fig. S4). After inoculation with ARP1, a strain with the same morphological and molecular characteristics as the original isolate was re-isolated, confirming Koch's postulates. To our knowledge, this is the first report of P. palmivora causing root and stem rot of A. vera in the study region. This disease could be a potential risk for aloe production and therefore appropriate management measures should be taken.

Bacillus amyloliquefaciens AK-12 Helps Rapeseed Establish a Protection against Brevicoryne brassicae.

Aphids are a serious threat to rapeseed (Brassica napus L.) production, and cause unmanageable loss. Therefore, effective prevention and management strategies are urgently required to avoid losses. Bacillus amyloliquefaciens AK-12 isolated from a dead aphid with aphicidal activity was tagged with a green fluorescent protein through a natural transformation. The transformed strains were checked for stability and growth, and the best-performing strain was tested for its colonization inside and outside the rapeseed plant. The stability of AK-12-GFP reached more than 95%, and the growth curve was consistent with that of AK-12. After 30 days of treatment, the colonization of 1 × 106 CFU/g was recorded in rapeseed leaves. Interestingly, AK-12 reduced the aphid transmission rate compared with the control and improved the growth of the rapeseed seedlings. Meanwhile, the AK-12 strain also exhibited phosphorus, potassium-solubilizing, and nitrogen-fixing activity, and produced 2.61 µg/mL of IAA at 24 h. Regulation in the activity of four enzymes was detected after the AK-12 treatment. Phenylalanine ammonia lyase (PAL) was recorded at a maximum of 86.84 U/g after 36 h, and catalase (CAT) decreased after 48 h; however, peroxidase (POD) and polyphenol oxidase (PPO) reached the maximum within 12 h of AK-12 application. Additionally, important resistance genes related to these enzymes were upregulated, indicating the activation of a defense response in the rapeseed against aphids. In conclusion, defense enzymes and defense-related gene activation could improve the pest resistance in rapeseed, which has good application prospects for the future to be developed into biopesticide.

The hidden treasures of citrus: finding Huanglongbing cure where it was lost.

Huanglongbing (HLB), a deadly citrus disease which has significantly downsized the entire industry worldwide. The intractable and incurable disease has brought the citriculture an enormous loss of productivity. With no resistant varieties available, failure of chemical treatments despite repeated applications, and hazardous consequences to environmental health, have led to large-scale research to find a sustainable cure. Inside plants, the key determinants of health and safety, live the endophytic microbes. Endophytes possess unrivaled plant benefiting properties. The progression of HLB is known to cause disturbance in endophytic bacterial communities. Given the importance of the plant endophytic microbiome in disease progression, the notion of engineering microbiomes through indigenous endophytes is attracting scientific attention which is considered revolutionary as it precludes the incompatibility concerns associated with the use of alien (microbes from other plant species) endophytes. In this review, we briefly discuss the transformation of the plant-pathogen-environment to the plant-pathogen-microbial system in a disease triangle. We also argue the employment of indigenous endophytes isolated from a healthy state to engineer the diseased citrus endophytic microbiomes that can provide sustainable solution for vascular pathogens. We evaluated the plethora of microbiomes responses to the re-introduction of endophytes which leads to disease resistance in the citrus host. The idea is not merely confined to citrus-HLB, but it is globally applicable for tailoring a customized cure for general plant-pathogen systems particularly for the diseases caused by the vascular system-restricted pathogens.

First Report of Phyllosticta capitalensis Causing Black Spot on Radermachera hainanensis Merr. in China.

Radermachera hainanensis Merr. plants are native in south-central and southeast of China. Plants produce large flowers, and are widely cultivated in China as ornamentals. In April 2020, R. hainanensis Merr. plants grown in Cixi Lvpin Garden (30°26'54″N, 121°25'48″E), Zhejiang Province, were found to have many black circular necrotic lesions. In the early infection stage, the lesions appeared in lower leaves as small black circular spots which developed later into large spots (11 to 38 mm diameter) with grey centers and chlorotic edges. Ultimately, the spots spread and merged. Moreover, infected leaves showed premature leaf fall. Disease intensity reached approximately 20% of plants in the affected field (0.5 ha). After effective chemical control, this disease did not spread to other healthy plants in the same garden. To identify the causative pathogen associated with the disease, ten symptomatic leaves were collected from ten different plants. Leaf tissues were cut from the lesion margins and sterilized as follows: surface sterilized with 75% ethanol for 30 seconds and washed three times in sterile distilled water. The leaf tissues were then dipped into 10% sodium hypochlorite for 3-4 minutes, then washed three times in distilled water and dried on a sterile filter paper. After drying, the surface-sterilized leaf discs were cut to small pieces (3×3 mm) and transferred to potato dextrose agar (PDA) plates and incubated at 28°C for 2 to 3 days under 12 h photoperiod. A total of 15 isolates were obtained from the affected leaves, and all the isolates displayed the same colony characteristics. Then, three single-spore isolates were randomly selected (F2, F5 and F8) for further study. The fungal colonies were dark green with a granular surface, and irregular white edges, later turning black. Conidia were one-celled, oval, and narrow at the end with a single apical end, measuring from 7.8 to 11.1 × 4.6 to 5.9 µm (av. 9.5 × 5.2 µm, n=50). These morphological characteristics were consistent with the description of Phyllosticta capitalensis (Wikee et al. 2013; Guarnaccia et al. 2017). The identity of three representative isolates were confirmed by a multilocus approach. The DNA of three isolates were extracted and partial sequences of ribosomal internal transcribed spacer (ITS), actin (ACT), and translation elongation factor 1-alpha (TEF1-α) were amplified and sequenced as previously described (White et al. 1990; O'Donnell et al. 1998; Carbone & Kohn et al. 1999). The three selected isolates shared 100% identical sequence of ITS, ACT and TEF1-α. Then representative isolate F8 was selected for further study. BLAST analysis in GenBank showed that the obtained sequence of ITS (MZ317550) had 99% identity to P. elongata isolate eSX25240811. Other two sequences of ACT (MZ326837) and TEF1-α(MZ326839) showed 99% and 98% identity to P. capitalensis isolate YLWB01, respectively. The phylogenetic trees were constructed by Bootstrap method with 1000 replications using Maximum Likelihood model implemented in the MEGA 7. Results showed that the isolate F8 clustered with P. capitalensis with 100% bootstrap support. Pathogenicity of strain F8 was tested by Koch's postulates. A pathogenicity test was performed in a greenhouse with 80% relative humidity at 28°C. 20 healthy plants were sprayed with a 1×106 conidia ml-1 suspension (three leaves from each individual plants) and another 20 healthy plants were sprayed with sterile distilled water (three leaves from each individual plant) as control. Conidia was obtained from PDA plates after 7 days of incubation in the biochemical incubator at 28°C and concentration was counted in hemacytometer. After 15 days, disease symptoms were observed on all inoculated leaves, whereas the control plants remained asymptomatic. After that, P. capitalensis was re-isolated only from the infected leaves and identified by morphological and sequence analyses. Early identification of P. capitalensis as a causal agent for black spot is crucial to employ effective disease management strategies to control disease in the field. P. capitalensis has been reported on many crops in China (Cheng et al. 2019; Tang et al. 2020; Liao et al. 2020). However, to our knowledge, this is the first report of black spot disease caused by P. capitalensis on Radermachera hainanensis Merr. in China.

First report of a new leaf blight disease of bletilla (Bletilla striata) caused by Curvularia reesii in China.

Bletilla striata is an important Chinese herbal plant grown widely in southwest China (Qian et al. 2021). Leaf blight was found on cultivated bletilla crops in Yunnan in 2021. The disease infected bletilla leaves and it was present in the field from April to November with the highest incidence (86% plants diseased) recorded in early September in Puer area. Foliar lesions were circular (Φ0.5-1.8 cm) or oval, with pale-gray center and narrow gray-brown outer area surrounded by a yellow halo. The lesions coalesced later to form large irregular spots or blighted areas on leaves. Symptomatic bletilla leaves were sampled from fields in Jiangcheng (E101.8672o, N22.5803o) and Simao (E109.7816o, N22.7891o) counties, Yunnan in July 2021. Seven fungal isolates were obtained from (BJ01-BJ04) and Simao samples (HBJ05-HBJ07) via lesion-tissue culture and hypha-tip purification on PDA medium. A pathogenicity test following Koch's Postulates (Grimms et al. 2006) was conducted using each isolate by inoculating 45-day old bletilla plant (n=30, Zihua cultivar) in a greenhouse through spraying hypha-spore suspension (3.25×104 CFU/mL) prepared with 14 d fresh DNA culture. Non-inoculated plants (n=30) were used as controls. The experiment was repeated once. The isolates BJ02 and HBJ06 (deposited in Yunnan Agric. Univ. Microbes Herbarium) were shown pathogenic to bletilla since similar lesions formed on seedlings 7 d post inoculation and pure fungal cultures with the same colony morphology as those of BJ02 and HBJ06 were re-isolated from leaf lesions 14 dpi. Isolates BJ02 and HBJ06 produced identical colony and conidium morphology after they were incubated at 25oC for 7 d on PDA. Colonies were circular, pale brown, Φ5.5-7.5cm, with villous surface and abundant aerial hyphae. Mycelia were septate, colorless, Φ3-4 µm and with acute-angled branches. Conidiophores developed from hyphae were erect, septate, pale-brown colored and 60-200 µm long. Conidia (produced scarcely and ripened slowly) were long-oval or petaloid, straight or slightly curved, brown, sized 28-45×10-14 µm. Most conidia were divided into 4 cells by 3 septa; the middle two were bigger than the basal and apex cells. Both BJ02 and HBJ06 were identified as Curvularia sp. based on their morphological characters (Tan et al. 2018). The rDNA-ITS, TEF1α and GAPDH genes (Tan et al. 2018) were amplified from these isolates with PCR (White et al. 1990) and sequenced. ITS sequences of the two isolates were both 574 bp (acc. no. OL587997 & OL336480) and 100% (574/574 bp) identical shown by blast comparison. Further blast analyses of ITS (574 bp, OL587997), TEF1α (532 bp, ON637120) and GAPDH (881 bp, ON637121) from isolate BJ02 showed that they were 99.27% (547/551 bp), 100% (842/842 bp) and 99.8% (507/508 bp) identical respectively with those of Curvularia reesii BRIP4358 (MH414907). The 3 genes of BJ02 were concatenated and phylogenic analysis (Tamura et al, 2013) of the concatenated sequence with those of Curvularia spp. showed that BJ02 was clustered with C. reesii BRIP4358 on the same end-branch of the tree with 100% confidence. Therefore, BJ02 and HBJ06 are the same species identified as Curvularia reesii and it is the pathogen causing bletilla leaf blight. C. reesii was first isolated from the air in Australia in 1963 and was named by Tan et al. in 2018. It has not been reported as a plant pathogen elsewhere. This is the first record of this fungus causing bletilla leaf blight in China. Keywords: Bletilla striata; leaf blight; Curvularia reesii; disease symptoms; pathogen morphology; multigene identification References (1) D.J. Grimes. Microbes, 1(5): 223-228, 2006. (2) L.H. Qian et al. Jiangshu Agric. Sci. 49(19): 64-71, 2021. (3) K. Tamura et al. Mol. Bio. & Evol. 30 (12): 2725- 2729, 2013. (4) Y. P. Tan et al. MycoKeys, 35: 1-25. 2018. (5) T.J. White et al. In: PCR Protocols: A Guide to Methods and Applications (eds. M.A. Innis et al.), Acad. Press, Inc. New York. 315-322, 1990.

First report of apple root rot caused by Phytopythium vexans in China.

Apple production is of great economic importance in the fruit industry of China, where Yunnan Province is considered as a major producing area. A survey was conducted to identify apple trees that were problematic from March to November 2020 in Yunnan Province. Symptoms included smaller yellowing leaves, fewer sprouts per branch, browning and necrosis of the roots and lower parts of the stem bark, and wilting. 20% to 45% of apple trees were found infected and randomly scattered in the surveyed orchards. A total of 110 soil samples were collected from the root area of symptomatic apple trees in Tuanjie Town of Kunming City, Zhaotong City, and Malong District of Qujing City in Yunnan Province. Two grams of each soil sample was suspended in 400 ml of sterile water for three days and each soil extract was baited with two apple leaves (Red Fuji's). Following the baiting, those leaves were cut into 10 pieces (5mm×5mm), surface-sterilized with 70% ethanol for 30 seconds, rinsed three times with sterile water, and then air-dried. Each leaf piece was placed in a Petri dish with the oatmeal agar medium containing PCNB 20 mg/ml, rifampicin 20 mg/ml, and then incubated at 25℃ in the dark for 3 days. A mycelial agar plug was picked from the edge of the colonies and transferred to a fresh Potato Dexrose Agar (PDA) plate. Seventy colonies with similar growing characteristics were isolated from the 110 soil samples. Three isolates were retained for further analysis and named XLD8-1, SD1, and YF2. After being cultivated on PDA plates and incubated at 25℃ in the dark for 4 days, their colonies were rose petal-type and white with dense aerial hyphae (Fig 1, A). In ten days of incubation, oogonium measuring 24.55 ± 1.9µm × 20.27 ± 2.3µm and sporangia measuring 21.65 ± 1.3µm × 19.35 ± 1µm were observed (Fig 1, C, D). The total DNA of the isolates was extracted and amplified using three pairs of primers, ITS1/ITS4 (White et al. 1990), LROR/LR7 (LSU) (Vilgalys R, et al. 1990), and FM58/FM66 (COXⅡ) (Martin F N. 2000). The sequences were uploaded to GenBank (Accession No. OL960234, OK037658, OK052604 for ITS, OL960388, OM838413, OM838314 for LSU, and OM962847, OM962848, OM962849 for COXⅡ). ITS sequences of the three isolates (XLD8-1, SD1, YF2) showed 99.87%,99.87%, 99.87% similar to Pp. vexans (Accession No. AB468784, AB468784, and AM701801). LSU sequences of the three isolates showed 99.92%, 99.72%, 100% similar to Pp. vexans (Accession No. EF426541, MT729990, and EF426541). COXⅡ sequences of the three isolates showed 100%, 99.81%, 99.81% similar to Pp. vexans (Accession No. GU133560). Based on the sequence similarity and morphology, the isolates were identified as Phytopythium vexans. Koch's postulates were conducted by wounding the bases of 3 apple seedlings (1-year-old Red Fuji's) with a cork borer. A plug of mycelium of the isolate XLD8-1 grown on PDA plates was placed on each wound (Fig 1, B). Controls were set up to use sterile agar plugs as an inoculum. Seedlings have incubated an incubator at 23-26°C under the alternating light and dark intervals, 12-hours of each. In 15 days, after were inoculated with XLD8-1 the roots and lower part of the stem bark of those seedlings became brownish and necrotic, and their epidermis was easily sloughed off (Fig 1, E-G). The pathogen isolated from the necrotic root tissues were identical to the isolate XLD8-1. Symptoms of apple growth decline caused by Pp. vexans were reported in Morocco (Jabiri Salma, et al. 2021). This experiment verified that Pp. vexans causes root rot of apple. In China, Fusarium sp. is usually considered the main pathogen causing apple root rot. However, the discovery of large numbers of apple trees that were infected by Pp. vexans in Yunnan Province and the confirmation of pathogenicity of Pp. vexans on apple seedlings have demonstrated for the first time that Pp. vexans could cause apple root rot as Fusarium spp does and become an incoming threat to the apple industry, which lays the foundation for study on the disease epidemiology and integrated management of apple root rot in China. References: Jabiri Salma, et al. 2021. Microorganisms, doi:10.3390/MICROORGANISMS9091916. Martin, F. N. 2000. Mycologia, 92(4), 711-727. Vilgalys R., et al. 1990. Journal of Bacteriology, 172:4238-4246 White, T. J., et al. 1990. PCR Protocols: a guide to methods and applications, 18: 315.

Phytophthora cinnamomi causing root rot on Rhododendron lapponicum and control it using potential biocontrol agents.

Rhododendron lapponicum (R. lapponicum) is a dwarf Rhododendron species, which is severely infected with root rot and wilt in Yunnan province, China. However, the causal agent causing these symptoms was unknown. An isolate, Pci-1 was identified as Phytophthora cinnamomi, based on its morphology and the sequences of ß-tubulin, internal transcribed spacer, and Ypt1 genes and verified according to the Koch's postulate. We found that this pathogen could infect 14 species of plants, including Althaea rosea, Viburnum cylindricum, and Brassica napus. Strain Pci-1 could cause R. lapponicum to wither and die; and it grows best in an oat medium with pH 7.0 - 8.0 and an optimum temperature of 27°C. We suggest that the rhizosphere of R. lapponicum treated with biocontrol strains Paenibacillus polymyxoides P2-5 and Trichoderma asperellum Tv-1 showed a significant inhibitory effect on pathogen Pci-1. The inhibitory effect of 70% dimethomorph + cymoxanil was significantly higher with EC50 and EC90 values of 0.1894 and 0.3618 a.i. µg/ml, respectively. Greenhouse experiments revealed that the pathogen load is decreased in the presence of potential antagonists. This study provides fundamentals on risk assessment and theoretical support for the management of P. cinnamomi pathogen and contributes significantly to the planting of forest and horticultural crops in a disease-free environment.

Novel Virulence Factors Deciphering Klebsiella pneumoniae KpC4 Infect Maize as a Crossing-Kingdom Pathogen: An Emerging Environmental Threat.

Klebsiella pneumoniae is not only a human and animal opportunistic pathogen, but a food-borne pathogen. Cross-kingdom infection has been focused on since K. pneumoniae was identified as the pathogen of maize, banana, and pomegranate. Although the pathogenicity of K. pneumoniae strains (from ditch water, maize, and human) on plant and mice has been confirmed, there are no reports to explain the molecular mechanisms of the pathogen. This study uncovered the K. pneumoniae KpC4 isolated from maize top rot for the determination of various virulence genes and resistance genes. At least thirteen plant disease-causing genes are found to be involved in the disruption of plant defense. Among them, rcsB is responsible for causing disease in both plants and animals. The novel sequence types provide solid evidence that the pathogen invades plant and has robust ecological adaptability. It is imperative to perform further studies on the verification of these KpC4 genes' functions to understand the molecular mechanisms involved in plant−pathogen interactions.

Ecology and etiology of bacterial top rot in maize caused by Klebsiella pneumoniae KpC4.

Klebsiella pneumoniae is an important opportunistic pathogen in humans and animals. Recently, K. pneumoniae KpC4 was identified as a causative agent of bacterial top rot in maize, which has been observed in many areas of Yunnan province, China. KpC4 is potentially dangerous to humans and livestock due to its cross-kingdom infection ability. Our study revealed the disease cycle of maize bacterial top rot caused by KpC4 and the ecological adaptability and host range of KpC4. We found same pathogenicity in maize between KpC4, the environmental strains E1, E4 (K1 serotype), E5, and the clinical strain K. pneumoniae 138 (Kp138). Alternative hosts of K. pneumoniae include not only humans and animals but also a variety of plants (such as maize, banana and sorghum). One of the survival strategies of K. pneumoniae is ecological adaptability, which is an essential factor for KpC4 to be able to cause bacterial top rot in maize. K. pneumoniae, for example, could survive in large numbers (2.34 ± 0.22 × 103 cfu/g) not only in the maize leaves (2.34 ± 0.22 × 103 cfu/g) under natural light, but persist in dried maize plant debris (1.51 × 104 cfu/g) for at least 6 months. K. pneumoniae strains from different sources can generally induce infection in susceptible hosts. Thus, this study revealed the ecological basis of KpC4 cross-kingdom infections, laying the foundation for the study of the mechanisms underlying cross-kingdom infections involving this type of human/animal opportunistic pathogen.

First Report of Colletotrichum siamense Causing Anthracnose on Zinnia elegans Jacq. in China.

Zinnia elegans (syn. Zinnia violacea), known as common zinnia, is one of the most spectacular ornamental plants in the family Asteraceae. Zinnia plants are widely cultivated in China for their impressive range in flower colours and profuse bloom over a long period. In April 2019, Zinnia plants grown in Ningbo Botanical Garden (29°56'57″N, 121°36'20″E) were found to have many circular necrotic lesions. In the early infection stage, the lesions appeared as small circular specks which developed later into large spots (15 to 32 mm diameter). Typical symptoms appeared to be grayish white centers with a chlorotic edges and disease incidence reached approximately 80% of plants in the affected field. Moreover, the growth of Zinnia plants was seriously affected by the disease. To identify the causative pathogen associated with the disease, 10 symptomatic leaves were collected from ten different Zinnia plants. Leaf tissues were cut from the lesion margins, surface sterilized with 75% ethanol for 30 seconds and rinsed three times in sterile distilled water. The leaf tissues were then dipped into 10% sodium hypochlorite for 2-3 minutes, washed three times in distilled water and dried on a sterile filter paper. After drying, the surface-sterilized leaf discs were transferred to potato dextrose agar (PDA) plates and incubated at 28°C for 2 to 3 days under the 12 h photoperiod. A total of ten pure fungal isolates were obtained and all the isolates displayed the same colony structure. Afterwards, three pure strains were randomly selected (F1, F3 and F5) for further study. The fungal colonies showed gray to brownish aerial mycelia with pink-colored masses of conidia. Conidia were one-celled, hyaline, cylindrical to subcylindrical, spindle-shaped with obtuse ends, measuring from 15.6 to 17.3 × 4.6 to 5.1 µm with both ends rounded. These morphological characteristics were consistent with the description of Colletotrichum gloeosporioides complex (Weir et al. 2012). The identity of a representative isolate, F3, was confirmed by a multilocus approach. Genomic DAN of isolate F3 was extracted and partial sequences of actin (ACT), chitin synthase (CHS), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), ribosomal internal transcribed spacer (ITS), manganese-superoxide dismutase (SOD2) , glutamine synthatase (GS), beta-tubulin (TUB2) and calmodulin (CAL) were amplified and sequenced as previously described (Weir et al. 2012). These nucleotide sequences were deposited in GenBank (accession MN972436 to MN972440, and MT266559 to MT266561; all sequences in FASTA format are shown (Supplementary S1). BLAST analysis of ITS, ACT, CHS, GAPDH and GS sequences from the F3 isolate revealed similarity to C. gloeosporioides voucher strain ZH01 with 100%, 100%，99%, 99% and 99% identity, respectively. SOD, TUB2 and CAL sequences showed similarity to C. siamense with 100%, 100% and 100% identity, respectively. The phylogenetic trees were constructed by Maximum Likelihood method (ML) using JTT model implemented in the MEGA 7. Results inferred from the concatenated sequences (ACT, CHS, GAPDH, ITS, SOD, GS, TUB2 and CAL) placed the isolate F3 within the C. siamense cluster (Supplementary S2). To confirm pathogenicity of the fungus, Koch's postulates were conducted by spraying 20 Zinnia plants (60-day-old) with a 1 × 106 conidia/ml suspension. Plants were maintained in the growth chamber at 25°C and 85% relative humidity. After 10 to 15 days, symptoms were observed on all inoculated leaves and resembled those observed in the field, whereas the control plants remained asymptomatic. Here, C. siamense was isolated only from the infected Zinnia leaves and identified by morphological and gene sequencing analyses. C. siamense has been reported in many crops in China (Yang et al. 2019; Chen et al. 2019; Wang et al. 2019). However, to our knowledge, this is the first report of anthracnose caused by C. siamense on Zinnia elegans in China. References Chen, X., Wang, T., Guo, H., Zhu, P. K., and Xu, L. 2019. First report of anthracnose of Camellia sasanqua caused by Colletotrichum siamense in China. Plant Dis. 103:1423-1423. Wang, Y., Qin, H. Y., Liu, Y. X., Fan, S. T., Sun, D., Yang, Y. M., Li, C. Y., and Ai, J. 2019. First report of anthracnose caused by Colletotrichum siamense on Actinidia arguta in China. Plant Dis. 103:372-373. Weir, B. S., Johnston, P. R., and Damm, U. 2012. The Colletotrichum gloeosporioides species complex. Stud. Mycol. 73: 115-180. Yang, S., Wang, H. X., Yi, Y. J., and Tan, L. L. 2019. First report that Colletotrichum siamense causes leaf spots on Camellia japonica in China. Plant Dis. 103:2127-2127.

Huanglongbing Control: Perhaps the End of the Beginning.

Huanglongbing (HLB) is one of the most destructive citrus plant diseases worldwide. It is associated with the fastidious phloem-limited α-proteobacteria 'Candidatus Liberibacter asiaticus', 'Ca. Liberibacter africanus' and 'Ca. Liberibacter americanus'. In recent years, HLB-associated Liberibacters have extended to North and South America. The causal agents of HLB have been putatively identified, and their transmission pathways and worldwide population structure have been extensively studied. However, very little is known about the epidemiologic relationships of Ca. L. asiaticus, which has limited the scope of HLB research and especially the development of control strategies. HLB-affected plants produce damaged fruits and die within several years. To control the disease, scientists have developed new compounds and screened existing compounds for their antibiotic and antimicrobial activities against the disease. These compounds, however, have very little or even no effect on the disease. The aim of the present review was to compile and compare different methods of HLB disease control with newly developed integrative strategies. In light of recent studies, we also describe how to control the vectors of this disease and the biological control of other citrus plant pathogens. This work could steer the attention of scientists towards integrative control strategies.

Age influences the olfactory profiles of the migratory oriental armyworm mythimna separate at the molecular level.

BACKGROUND: The oriental armyworm Mythimna separata (Walk) is a serious migratory pest; however, studies on its olfactory response and its underlying molecular mechanism are limited. To gain insights to the olfactory mechanism of migration, olfactory genes were identified using antennal transcriptome analysis. The olfactory response and the expression of olfactory genes for 1-day and 5-day-old moths were respectively investigated by EAG and RT-qPCR analyses. RESULTS: Putative 126 olfactory genes were identified in M. separata, which included 43 ORs, 13 GRs, 16 IRs, 37 OBPs, 14 CSPs, and 3 SNMPs. RPKM values of IR75d and 10 ORs were larger than co-receptors IR25a and ORco, and the RPKM value of PR2 was larger than that of other ORs. Expression of GR1 (sweet receptor) was higher than that of other GRs. Several sex pheromones activated evident EAG responses where the responses of 5-day-old male moths to the sex pheromones were significantly greater than those of female and 1-day old male moths. In accordance with the EAG response, 11 pheromone genes, including 6 PRs and 5 PBPs were identified in M. separate, and the expression levels of 7 pheromone genes in 5-day-old moths were significantly higher than those of females and 1-day-old moths. PR2 and PBP2 might be used in identifying Z11-16: Ald, which is the main sex pheromone component of M. separata. EAG responses to 16 plant volatiles and the expression levels of 43 olfactory genes in 1-day-old moths were significantly greater than that observed in the 5-day-old moths. Heptanal, Z6-nonenal, and benzaldehyde might be very important floral volatiles for host searching and recognized by several olfactory genes with high expression. Some plant volatiles might be important to male moths because the EAG response to 16 plant volatiles and the expression of 43 olfactory genes were significantly larger in males than in females. CONCLUSIONS: The findings of the present study show the effect of adult age on olfactory responses and expression profile of olfactory genes in the migratory pest M. separate.

Whole-genome sequencing of Bacillus subtilis XF-1 reveals mechanisms for biological control and multiple beneficial properties in plants.

Bacillus subtilis XF-1 is a gram-positive, plant-associated bacterium that stimulates plant growth and produces secondary metabolites that suppress soil-borne plant pathogens. In particular, it is especially highly efficient at controlling the clubroot disease of cruciferous crops. Its 4,061,186-bp genome contains an estimated 3853 protein-coding sequences and the 1155 genes of XF-1 are present in most genome-sequenced Bacillus strains: 3757 genes in B. subtilis 168, and 1164 in B. amyloliquefaciens FZB42. Analysis using the Cluster of Orthologous Groups database of proteins shows that 60 genes control bacterial mobility, 221 genes are related to cell wall and membrane biosynthesis, and more than 112 are genes associated with secondary metabolites. In addition, the genes contributed to the strain's plant colonization, bio-control and stimulation of plant growth. Sequencing of the genome is a fundamental step for developing a desired strain to serve as an efficient biological control agent and plant growth stimulator. Similar to other members of the taxon, XF-1 has a genome that contains giant gene clusters for the non-ribosomal synthesis of antifungal lipopeptides (surfactin and fengycin), the polyketides (macrolactin and bacillaene), the siderophore bacillibactin, and the dipeptide bacilysin. There are two synthesis pathways for volatile growth-promoting compounds. The expression of biosynthesized antibiotic peptides in XF-1 was revealed by matrix-assisted laser desorption/ionization-time of flight mass spectrometry.

Amylocyclicin, a novel circular bacteriocin produced by Bacillus amyloliquefaciens FZB42.

Bacillus amyloliquefaciens FZB42 is a Gram-positive plant growth-promoting bacterium with an impressive capacity to synthesize nonribosomal secondary metabolites with antimicrobial activity. Here we report on a novel circular bacteriocin which is ribosomally synthesized by FZB42. The compound displayed high antibacterial activity against closely related Gram-positive bacteria. Transposon mutagenesis and subsequent site-specific mutagenesis combined with matrix-assisted laser desorption ionization-time of flight mass spectroscopy revealed that a cluster of six genes covering 4,490 bp was responsible for the production, modification, and export of and immunity to an antibacterial compound, here designated amylocyclicin, with a molecular mass of 6,381 Da. Peptide sequencing of the fragments obtained after tryptic digestion of the purified peptide revealed posttranslational cleavage of an N-terminal extension and head-to-tail circularization of the novel bacteriocin. Homology to other putative circular bacteriocins in related bacteria let us assume that this type of peptide is widespread among the Bacillus/Paenibacillus taxon.

Enhancement of Biocontrol Activities and Cyclic Lipopeptides Production by Chemical Mutagenesis of Bacillus subtilis XF-1, a Biocontrol Agent of Plasmodiophora brassicae and Fusarium solani.

Bacillus subtilis XF-1 has been used as a biocontrol agent of clubroot disease of crucifers infected by Plasmodiophora brassicae, an obligate pathogen. In order to maximize the growth inhibition of the pathogen, random mutagenesis using N-methyl-N'-nitro-N-nitrosoguanidine was applied to strain XF-1. The efficacy of 226 selected mutants was assessed against the growth of an indicator fungal pathogen: Fusarium solani using agar plate assay and the disruptive effects on the resting spores of P. brassicae. Four mutants exhibited inhibition activity significantly higher than the wild type. The cell extracts of these mutants and the XF-1 were subjected to matrix-assisted laser desorption ionization-time of flight mass spectra analysis, and three families of cyclic lipopeptides (CLPs) fengycin, surfactin and iturin were identified from the parental strain and the screened mutants. However, the relative contents and compound diversity changed after mutagenesis, and there was slight variation in the surfactin and fengycin. Notably, only 5 iturin components were discovered from the wild strain XF-1, but 13 were obtained from the mutant strains, and the relative CLPs contents of all mutant strains increased substantially. The results suggested that CLPs might be one of main biocontrol mechanisms of the clubroot disease by XF-1. The 4 mutants are far more effective than the parental strain, and they would be promising biocontrol candidates not only against P. brassicae but probably other plant diseases caused by fungi.